Serotonin (5-HT) and related agonists are well characterized to act upon the 2C isoform of its receptor (5HT2CR) in the arcuate and paraventricular nuclei of the hypothalamus (ARH and PVH) to suppress food intake and exert anti-obesity effects. The specific neuronal targets of 5-HT and related agonists in the ARH and PVH required for such effects are, in contrast, not well defined. Recent work in which 5HT2CR expression was selectively restored to pro-opiomelanocortin (POMC) neurons within the ARH identifies that this distinct neuronal sub-set is a strong candidate to be a target. This notion is based on evidence that selective re- activation of 5HT2CRs in POMC neurons rescues the hyperphagic and obese phenotype of 5HT2CR-null mice. Preliminary data in this application also highlights that simple-minded 1 (SIM1) neurons in the PVH co-express 5HT2CRs and may be one type of neuron targeted by 5-HT and related agonists. The current proposal aims to test whether 5-HT and related agonists require 5HT2CR expression in POMC and SIM1 neurons in the ARH and PVH, respectively, to maintain normal body weight and to exert anti-obesity effects. This will be accomplished by combining genetically engineered mouse models in which endogenous 5HT2CR are selectively deleted in either POMC or SIM1 neurons with comprehensive analyses of energy homeostasis and administration of 5-HT agonists m-chloro-phenylpiperazine (mCPP) and D-fenfluramine (d-Fen). The proposed mouse models in which 5HT2CRs will be selectively deleted using cre-lox techniques in either POMC or SIM1 neurons provide a powerful and unique model to assess the physiological relevance and requirement for these pathways. The comprehensive approach to analyze the phenotype of these mice will include histological validation of the mouse model as well as assessments of brain development and neural survival. Metabolic cages studies will also be performed in mice fed chow and high-fat diet to assess food intake, energy expenditure, and physical activity. Taken together, these approaches will provide mechanistic insight into the effects of 5-HT and related agonists in the CNS. The expected findings are that 5-HT and related agonist will require POMC and SIM1 neurons to maintain normal body weight homeostasis and to fully mediate the anti- obesity effects of mCPP and d-Fen. These findings would improve our understanding about how the body controls energy homeostasis and potentially offer new therapeutic targets.